Generally, a spark plug for use in an internal combustion engine, such as an automotive engine, is configured to ignite an air-fuel mixture supplied into a combustion chamber of the internal combustion engine, through generation of sparks across a spark discharge gap between a center electrode and a ground electrode.
In recent years, in order to cope with exhaust gas regulations and to improve fuel economy, lean-burn engines, direct-injection engines, low-emission engines, and like internal combustion engines have been actively developed. These internal combustion engines require a spark plug higher in ignition performance than conventional spark plugs.
A known spark plug having excellent ignition performance has a ground electrode on which a protrusion is provided. An example of such a spark plug is configured such that a noble metal tip of an iridium alloy, a platinum alloy, or the like, which exhibits excellent erosion resistance, is welded to the ground electrode, thereby forming the protrusion. For example, see Japanese Patent Application Laid-Open (kokai) No. 2003-317896, hereinafter “Patent Document 1”).
However, a noble metal tip of an iridium alloy, a platinum alloy, or the like is expensive. Thus, manufacturing cost may increase.
Thus, there is proposed a technique for working on the ground electrode itself so as to form the protrusion made of the same material as that used to form the ground electrode. For example, see Japanese Patent Application Laid-Open (kokai) No. 2006-286469 “Patent Docuement 2”).